1. Field of the Invention
The invention relates to a belt fixing unit, an image forming apparatus incorporating the fixing unit, and a toner material suitably used in the fixing unit capable of obviating undesirable fixing belt deformation and concomitant results of partial underdevelopment of electrophotographic images.
2. Discussion of the Background
As to an image fixing process undertaken in an image forming apparatus making use of electrophotography and electrostatic recording, a heat roller fixing unit is known, including a fixing roller having an internal heat source and a pressing roller pressed against the fixing roller. An image transfer sheet carrying an unfixed toner image is conveyed through a fixing nip between the fixing roller and the pressing roller, whereby the toner image is fixed onto the transfer sheet. The toner fixation is therefore carried out by heat transferred from the fixing roller and pressure applied by these rollers.
In the image fixing process, a toner, comprising at least resin materials, is melted to be fixed through either contacting the surface or penetrating into the texture of a transfer sheet under constant pressure.
It is known that the quality of fixed images depends more on the degree of toner melting than on the applied pressure. That is, toner melting conditions primarily determine the resulting quality of a fixed image.
As mentioned above, an unfixed toner image is conveyed in a conventional fixing unit through a fixing nip between the fixing roller and the pressing roller. The fixing nip comprises a pressurized portion at a high temperature generated by a heated roller brought to a close contact with a pressing roller, whereby the process of melting the toner and applying a pressure is performed while conveying a transfer sheet through the fixing nip.
In the fixing process utilized in a conventional image forming apparatus, the sheet conveyance speed generally ranges from 100 to 500 mm/s with the fixing nip width ranging from 4 to 10 mm.
In the heat roller fixing unit, a roller fixing method has been used in which a pressing roller formed with an elastic member on its surface roller is brought into contact with a fixing roller provided internally with a heat source.
Since the fixing nip in the roller fixing method is formed with the rollers contacting each other, the length of the fixing nip is inherently limited, and the linear speed of the fixing and the pressing rollers is relatively slow. Therefore, this method is not suitable for use in a high speed image forming apparatus.
Besides the roller fixing method, a belt fixing method has recently come into widespread use as exemplified by Japanese Laid-Open Patent Application No. 10-307496.
In contrast to the roller fixing method having a limited fixing nip length, the belt fixing method offers the advantage of a wider nip length as an improved feature among other important requirements of the image fixing process.
This fixing nib length corresponds to a range in which a toner is melted to be pressed to a transfer sheet under high temperature and pressure. In order to increase the nip width over a certain level in the roller fixing method, the roller diameter has to be larger, undesirably increasing the overall size of the image forming apparatus.
In the roller fixing method, by contrast, the fixing nip can be formed by utilizing a flexible belt wound around a roller having a relatively small diameter. As a result, a fixing nip having a sufficient width can be formed even in a small space in the apparatus.
The belt fixing method, however, has a drawback, as discussed next.
In a relatively small image forming apparatus of low productivity, small diameter rollers are suitably used. When the image forming apparatus and its units, such as the belt fixing unit in particular, are kept on a stand-by state for a prolonged period time, the fixing belt experiences high tensile stresses, and portions of the belt disposed along respective circumferential surfaces of the rollers may experience plastic deformation.
If the deformation is not easily restored after the system resumes operation, this deformation may cause several problems such as, for example, the irregularity in the rotation of the fixing belt and in the adherence to the heating roller. This may result in uneven heat conduction and concomitant results of partial underdevelopment among others.
This difficulty may seemingly be avoidable by increasing the tensile force applied to the fixing belt. Since the deformation can be released with more ease by increasing the tensile force, this measure may be allowed to some extent. In order to increase the tensile force, the members supporting the fixing belt, such as heating roller, fixing roller and others, also have to be reinforced accordingly.
Such reinforcement, however, will result in the increase not only in the volume of the rollers but also in heat capacity of the apparatus as a whole. As a result, since the time for heating increases, not only accessibility and serviceability worsen, but, the overall energy consumption of the apparatus increases.
It is desirable, therefore, to provide a belt fixing unit, an image forming apparatus incorporating the fixing unit, and a toner material suitably used in the fixing unit capable of obviating undesirable fixing belt deformation and partial underdevelopment of electrophotographic images.